Methyl Chloroform Continues to Constrain the Hydroxyl (OH) Variability in the Troposphere

Trends and variability in tropospheric hydroxyl (OH) radicals influence budgets of many greenhouse gases, air pollutant species, and ozone depleting substances. Estimations of tropospheric OH trends and variability based on budget analysis of methyl chloroform (CH3CCl3) and process-based chemistry transport models often produce conflicting results. Here we use a previously tested transport model to simulate atmospheric CH3CCl3 for the period 1985-2018. Based on mismatches between model output and observations, we derive consistent anomalies in the inverse lifetime of CH3CCl3 (K-G) using measurements from two independent observational networks (National Oceanic and Atmospheric Administration and Advanced Global Atmospheric Gases Experiment). Our method allows a separation between physical (transport, temperature) and chemical (i.e., abundance) influences on OH + CH3CCl3 reaction rate in the atmosphere. Small increases in K-G due to physical influences are mostly driven by increases in the temperature-dependent reaction between OH and CH3CCl3 and resulted in a smoothly varying increase of 0.80% decade(-1). Chemical effects on K-G, linked to global changes in OH sources and sinks, show larger year-to-year variations (similar to 2%-3%), and have a negative correlation with the El Nino Southern Oscillation. A significant positive trend in K-G can be derived after 2001, but it persists only through 2015 and only if we assume that CH3CCl3 emissions decayed more slowly over time than our best estimate suggests. If global CH3CCl3 emissions dropped below 3 Gg year(-1) after 2015, recent CH3CCl3 measurements indicate that the 2015-2018 loss rate of CH3CCl3 due to reaction with OH is comparable to its value 2 decades ago.

Automated summary

The study uses observations and simulation models to derive anomalies in the inverse lifetime of CH3CCl3 in the troposphere, revealing that the reaction rate of CH3CCl3 in the atmosphere is influenced by both physical and chemical factors. It explains that the changes in K-G are mainly driven by global variations in OH sources and sinks and chemical effects, showing significant positive trends and year-to-year fluctuations.